In an image sensing system having an image sensor that samples the image at discrete sampling locations, it is desirable to band limit the spatial frequency of the input image to reduce the appearance of spurious spatial frequency components in an image reconstructed from the signals produced by the image sensor. The production of the spurious spatial frequency components is called aliasing error. The effect occurs when the image includes spatial frequency components that are higher than one-half of the spatial sampling frequency of the sensor. Visual patterns produced by aliasing errors are often called Moire patterns. To this end, it is known to employ an optical spatial frequency filter in the optical path of an image sensing system.
U.S. Pat. No. 3,588,224 issued June 28, 1971 to D. H. Pritchard discloses an optical spatial frequency filter for use with a color TV camera tube having a striped color filter. The optical spatial frequency filter includes at least one pair of birefringent elements with a quarter wave plate positioned between them.
The first birefringent element in the filter splits an incoming ray of light into two spaced apart rays linearly polarized perpendicular to each other. The quarter wave plate changes the polarization state of the light rays from linearly to circularly polarized by retarding one component of the light waves with respect to an orthogonal component by a quarter wavelength. The second birefringent element splits each of the two circularly polarized rays into two spaced apart rays to form an array of four spaced apart rays.
When the optical spatial frequency filter is placed in the optical path of the image sensing system, each spot in the input image is split into four spaced apart spots on the image sensor. The image sensor is presented with four overlapping identical images, each image slightly displaced from the others. The resulting composite image presented to the image sensor appears as a slightly blurred version of the original image, having reduced spatial resolution. In one arrangement of the elements of the spatial frequency filter, the spaced apart spots lie along a line, thereby limiting the spatial resolution in the direction of the line. Alternatively, the elements can be arranged relative to each other to cause the spaced apart spots to lie on the corners of a parallelogram, thereby limiting the spatial resolution in two orthogonal directions. Further birefrengent elements and wave plates may be added to the filter to further modify its frequency response. In other spatial frequency filters of this type, the means for changing the polarization state of the light between the birefringent elements is an optically active element that directly rotates the polarization of the light, rather than retarding one component of it with respect to another.
In some color image sensors, the spatial sampling frequency in one color is different from the spatial sampling frequency of another color. For example, in one type of image sampling system, the spatial sampling frequency in green is twice the spatial sampling frequency in red or blue to account for the greater sensitivity of the human eye to green detail. To date, the spatial frequency filters described above have been built to function in an achromatic fashion, i.e. the spatial frequency response of the filters is substantially the same for all colors.
An achromatic frequency response presents a dilemma for use with a color image sensor where the spatial sampling frequency is higher for one color than it is for another. If the frequency response of the filter is optimally chosen to reduce aliasing in the more highly sampled color (e.g. green), the spatial frequencies in the other less highly sampled colors (e.g. blue and red) will not be adequately limited, resulting in aliasing in these colors. On the other hand, if the frequency response of the spatial filter is optimized for the less highly sampled colors (e.g. red and blue), the spatial frequencies in the more highly sampled color (e.g. green), will be unduly limited, and the sharpness of an image reconstructed from the samples will be undesirably reduced.